The roles of "A" cells as effector cells in tumor cell growth regulation and as accessory cells in antibody synthesis are to be studied in light of their presence in the light density PE subpopulation that is enriched for both these activities. "A" cells will be characterized as either possible precursor cells for macrophages or as a unique cell type. The studies will include the examination of the "A" cells for surface markers common for macrophages and lymphocytes as well as for antigenic and functional markers, and for their ability to differentiate into macrophages in culture. In addition, bone-marrow derived macrophage colonies will be examined for the presence or absence of "A" cells in an effort to determine whether macrophages and "A" cells share a common stem cell.